5.4 Gravitational Fields Flashcards

1
Q

What causes gravitational fields?

A

Any object with mass

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2
Q

How do gravitational field lines represent gravitational fields? (3)

A

At right angles to object
Arrows show direction of force (always attractive)
Separation of lines shows strength of field

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3
Q

Define gravitational field strength

A

The gravitational force exerted per unit mass at that point in the gravitational field

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4
Q

How is gravitational field strength and distance related?

A

Gravitational field strength is inversely proportional to the distance from the centre of mass of the object

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5
Q

What is assumed about the Earth’s gravitational field strength?

A

It is constant close to the surface of the Earth as the field is considered to be uniform

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6
Q

What does Newton’s law of gravitation assume? (2)

A

Point masses

Mass is concentrated at centre of mass

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7
Q

Why is there a negative sign in Newton’s law of gravitation?

A

Gravity is always an attractive force

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8
Q

What is Kepler’s first law of planetary motion?

A

The orbit of a planet is an ellipse with the sun at one of the two foci

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9
Q

How is a gravitational field shown to be uniform?

A

Equidistant, parallel field lines

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10
Q

State Newton’s law of gravitation

A

The force between two point masses is directly proportional to the product of the masses and inversely proportional to the square of their separation

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11
Q

What is eccentricity?

A

A measure of how elongated a circle is

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12
Q

What is Kepler’s second law of planetary motion?

A

A line segment joining a planet and the Sun sweeps out equal areas during equal intervals of time

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13
Q

What is Kepler’s third law of planetary motion?

A

The square of the orbital period of a planet is directly proportional to the cube of its average distance from the Sun

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14
Q

How is Kepler’s third law derived? (3)

A

Model the planetary orbit as a circle
Centripetal force on planet = gravitational force on planet
Determine speed by dividing circumference by orbital period

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15
Q

What is a geostationary satellite?

A

A satellite that stays above the same area of Earth

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16
Q

What are the conditions for a geostationary satellite? (4)

A

36000km from Earth’s surface
Orbit above the equator
Time period of 24 hours
Orbit in the same direction as Earth’s rotation

17
Q

What are polar orbits used for? (2)

A

Mapping

Reconnaissance

18
Q

What are Low Earth Orbits used for? (2)

A

Photography

Weather

19
Q

What are Medium Earth Orbits used for?

20
Q

What are geostationary orbits used for? (2)

A

Communications

TV

21
Q

What are High Earth Orbits used for?

22
Q

Define gravitational potential

A

The work done per unit mass to move an object to a point in a gravitational field from infinity

23
Q

What does infinity refer to in the definition of gravitational potential?

A

A distance so far from the gravitational field that its field strength is zero

24
Q

What is true about gravitational potential at infinity?

A

It is zero

25
Why are all values of gravitational potential negative? (2)
All masses attract each other | Work must be done to separate objects gravitational potential is zero at infinity
26
What does a graph of gravitational potential against distance look like? (3)
Most negative value at surface of object (graph starts here) Inversely proportional relationship All values of gravitational potential are negative
27
How does gravitational potential change when masses move? (2)
Moving towards a point mass decreases gravitational potential (more negative) Moving away from a point mass increases gravitational potential (less negative)
28
What does the area under a force-distance graph for a point mass represent?
Work done
29
What is escape velocity? (2)
The minimum velocity required to escape the gravitational field of a mass When loss in KE = gain in GPE